The present invention relates to an apparatus for calculating torque generated by an induction motor (hereinafter referred to as an "induction machine") driven by a PWM inverter.
According to the prior art, torque generated by an induction machine is obtained for controlling the induction machine from the vector product of the primary magnetic flux vector and the primary current vector of the induction machine as described in "Torque Limited Control for General Purpose Inverter" (1990 NATIONAL CONVENTION RECORD I.E.E. JAPAN, No. 579).
According to this method, however, since the method does not consider an iron loss of the induction machine, a large error may occur in the calculated result in the boosting state in that a reference voltage larger than usual is set with respect to a reference angular frequency, i.e. V/F setting exceeding the rated excitation.
As explained above, torque that an induction motor generates is obtained from the vector product of the primary magnetic flux vector and the primary current vector of the induction machine as described by the following equation (1). Also, the primary magnetic flux vector is calculated based on the primary voltage vector, primary current vector and primary resistance value, as shown by the equation (2). EQU .tau.=.phi..sub.1.times.i.sub.1 (1)
.phi..sub.1 =.intg.(v.sub.1 -r.sub.1.multidot.i.sub.1).multidot.dt (2)
The quantities in equations (1) and (2) are as follows.
i.sub.1 : primary current vector PA1 .phi..sub.1 : primary magnetic flux vector PA1 v.sub.1 : primary voltage vector PA1 r.sub.1 : primary resistance value PA1 f: primary frequency PA1 B.sub.m : maximum value of magnetic flux density PA1 .sigma..sub.h : constant determined by a core material PA1 .sigma..sub.e : constant determined by resistivity of a core PA1 d: thickness of the core
The following equation (3) is obtained by solving equations (1) and (2) on the rotating coordinate system (d-q axes coordinate system), rotating with the rotor of the induction machine, in the steady state. ##EQU1##
In equation (3), .omega..sub.1 represents a primary angular frequency. The following equations (4a) and (4b) are obtained from equation (3). EQU .phi..sub.1d =(v.sub.1q -r.sub.1.multidot.i.sub.1q)/.omega..sub.1 (4a) EQU .phi..sub.1q =(r.sub.1.multidot.i.sub.1d -v.sub.1d)/.omega..sub.1 (4b)
The numerator on the right side of equation (3) is a formula for obtaining electric power by subtracting the copper loss from electric power inputted to the induction machine. Therefore, this formula does not include so-called iron loss, that is the loss caused by the stator core.
In other words, since equation (3) does not consider the iron loss, a large error may occur in the calculated result in the boosting state in that a reference voltage larger than usual is set with respect to a reference angular frequency, i.e. V/F setting exceeding the rated excitation.
In view of the foregoing, it is an object of the invention to provide a calculating apparatus that facilitates calculating the torque generated by an induction motor in considering the iron loss.